1. Field of the Invention
The present invention relates to PID controllers. More specifically, the present invention is directed to a method of determining PID parameters for a PID feedback controller to produce optimum PID control on a controlled process and an automatic tuning controller utilizing the method.
2. Description of the Prior Art
In a conventional feedback process control system, a proportional-plus-integral-plus-derivative (PID) operation is performed with respect to a deviation (e) between a set point (sp) and a controlled variable (pv) fed back from the process, and the result of the PID operation is supplied as a control signal to the process. In order to perform an optimum control of the process, it is necessary that the PID parameters for performing the respective PID operation are set to their optimum values. Conventionally, the PID parameters have been manually adjusted. For implementing the manual adjustment, a step response method and a marginal sensitivity method have been well-known. In both the methods, however, it takes a long time for the measurement of characteristic, and the process control is stopped while the measurement is effected so that the value of pv obtained at that time cannot be the most desirable one.
On the other hand, methods have been proposed in which a nonlinear characteristic is provided in a PID controller so as to generate a limit cycle in a process. In those methods, a controller is arranged in a tuning mode and a nonlinear element is introduced in a signal path so as to effect a discontinuous control operation with respect to a deviation (a two-position control is a typical one). Upon generation of a limit cycle, it has been easy to obtain a characteristic of a process and the optimum parameters for the process on the basis of the waveform of the limit cycle.
In such a method as described above, however, if the values of the two positions of the nonlinear element are large, the range of fluctuation in the control signal corresponding to the nonlinear values becomes so large that the method cannot be used except for some thermal control systems or the like in which a fast response is not required.
On the other hand, the limit cycle has the equilibrium point (sp=pv) of the process as an operating basic point. Accordingly, if the values of the two positions were made small, sometimes the limit cycle could not be generated when a disturbance was generated in the process or the sp changed widely. Furthermore, at that time, the pv undesirably stayed at a point comparatively far from the sp. Consequently, such a conventional controller was inherently inadequate for practical use.